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1. Model description

Global Warming Projection Vol. 7 summarizes detailed climate change projection around Japan based on results from the Coupled atmosphere-ocean Regional Climate Model (CRCM) developed by the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA). The CRCM is forced by the MRI Coupled General Circulation Model version 2.3.2 (CGCM2.3.2). The results from the CGCM2.3.2 were provided for the IPCC Fourth Assessment Report.

1.1 MRI-CRCM : Coupled atmosphere-ocean Regional Climate Model

The CRCM is a high-resolution Coupled atmosphere-ocean Regional Climate Model to project climate change around Japan using sophisticated atmosphere-ocean interaction. It couples the 20-km-mesh Regional Climate Model (RCM20) and the North Pacific Ocean General Circulation Model (NPOGCM).

For further details of the CRCM, please refer to Sasaki et al. (2006).

(1) MRI-RCM20: Regional Climate Model with a 20-km mesh size

The RCM20 is a high-resolution atmospheric model for projecting climate change around Japan. It has a horizontal resolution of 20 km and 36 vertical levels.

The model has been improved from the previous version used in Global Warming Projection Vol. 6 (JMA, 2005), primarily in terms of a number of physical processes and the method of setting boundary conditions for coupling the ocean model, although its fundamental framework has been retained.

For further details of the RCM20, please refer to Sasaki et al. (2005).

(2) MRI-NPOGCM : North Pacific Ocean General Circulation Model

The ocean component of the CRCM is the North Pacific Ocean General Circulation Model (NPOGCM), which is based on the Meteorological Research Institute Community Ocean Model (MRI.COM) described by Ishikawa et al. (2005).

The NPOGCM has a domain from 15°S to 65°N and from 100°E to 75°W in the Pacific Ocean, with a horizontal resolution of 1/4° (longitude) by 1/6° (latitude) and 48 vertical levels. It is categorized as an eddy-permitting ocean model.

The open boundaries of the domain are replaced by vertical walls, i.e., there is no inflow/outflow at the boundaries of the Bering Strait, the Indonesian passage and the South Pacific Ocean. At the southern boundary, temperature and salinity are nudged to the monthly CGCM2.3.2 data to maintain the basic state of density stratification for the whole domain.

The physical factors driving the NPOGCM are wind stress (momentum flux), heat flux, fresh water flux (precipitation and evaporation), and longwave and shortwave radiation. The bulk formula is based on that of Kondo (1975), and sea-surface salinity is given by the results from the CGCM2.3.2.

For further details of the NPOGCM, please refer to Sato et al. (2006).

1.2 MRI-CGCM2.3.2 : Coupled General Circulation Model version 2.3.2

The CGCM2.3.2 is the latest version of the Coupled General Circulation Model developed by MRI. It has been improved from the previous version (the MRI-CGCM2.0), primarily in terms of its cloud scheme and other physical parameterizations, although the fundamental framework has been retained.

The atmospheric component has T42 horizontal resolution (with a transform grid size of approximately 280 km) and 30 vertical levels, with the top at 0.4 hPa.

The oceanic component is a grid point model. The horizontal resolution is 2.5° in longitude and 2.0° in latitude to the north/south of 12°N/S, with finer resolution up to 0.5° near the equator. Twenty-three vertical levels are unevenly placed between the surface and the deepest point of 5,000 m.

The CGCM2.3.2 uses flux correction for heat and fresh water to couple the atmosphere and ocean. It also adjusts wind stress in the equatorial region (12°S - 12°N). The improvements in the model's latest version have enabled the reproduction of a realistic climate without using flux correction. However, a version of the model with flux correction was used because we believe that it is more important to simulate basic climatic states as close to the actual present-day conditions as possible, since the results are used in studies on a regional scale.

The CGCM2.3.2 yields 3.2°C for the equilibrium climate sensitivity (the global average surface warming following a doubling of carbon dioxide concentration), which is larger than that in the former version (2.0°C). According to the IPCC Fourth Assessment Report, the current generation of GCMs covers a range of equilibrium climate sensitivity from 2.1°C to 4.4°C (with a mean value of 3.2°C).

For further details of the CGCM2.3.2, please refer to Yukimoto et al. (2006a).

1.3 Relationships among models (Fig A1.1)

The CRCM is forced by the CGCM2.3.2. To calculate the atmospheric part, two-step nesting is applied due to the coarse grid size of the CGCM2.3.2. The 60-km-mesh Regional Climate Model (RCM60) is nested within the CGCM2.3.2 results, and the RCM20 is nested within the RCM60 results.

The sea surface temperature calculated by the NPOGCM is used for the lower boundary condition of the RCM20, and the atmospheric surface data calculated by the RCM20 drive the NPOGCM. The data are exchanged between the NPOGCM and the RCM20 every hour during calculation. The variables transferred from the RCM20 to the NPOGCM are sea-level pressure, surface wind, surface temperature, surface specific humidity, precipitation amount, and downward shortwave and longwave radiative fluxes. The NPOGCM calculates surface fluxes using these atmospheric data. Outside the domain of the RCM20, the NPOGCM is driven by the atmospheric data of the RCM60 and the CGCM2.3.2.

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